in reference to a letter sent by the Pope to all the bishops, 1640s, from Late Latin encyclicus, from Latin encyclius, from Greek enkyklios "in a circle, circular" (see encyclopedia). As a noun, from 1837.

pastoral letter written by the pope for the whole Roman Catholic church on matters of doctrine, morals, or discipline. Although formal papal letters for the entire church were issued from the earliest days of the church, the first commonly called an encyclical was Ubi primum, dealing with episcopal duties, published by Benedict XIV in 1740. Only from the time of Pius IX (1846-78) have encyclicals been frequently used. Encyclicals are normally addressed to the bishops of the church, but a few (notably Pacem in terris by John XXIII) have been addressed also to "all men of good will." The formal title of an encyclical consists of the first few words of the official text; the language is usually Latin, and the document is not considered to be infallible.